Microstrip patch antennas are popular because they are generally small and light, relatively easy to fabricate, and with the proper feeding/receiving network, can transmit/receive beams of various polarizations. The small size and light weight of microstrip patch antennas are particularly advantageous for satellite applications in which such parameters directly affect project costs (such as the cost to launch a satellite into orbit).
Patch antennas which transmit and/or receive signals which are circularly polarized, as opposed to linearly polarized, are particularly useful in satellite communication systems. Linear polarization requires that an earth station tightly align its frame of reference with that of a satellite in order to achieve acceptable communications. Furthermore, as linearly polarized radiation propagates through the earth's atmosphere, its orientation tends to change thus making the earth-satellite alignment difficult to maintain. Circularly polarized radiation is less affected by such considerations. However, to achieve satisfactory communications, the degree of circular polarization (as measured by axial ratio) should be relatively high over a relatively broad bandwidth.
The bandwidth of a directly fed microstrip patch antenna is generally narrow (compared to, for example, a standard horn antenna), due at least in part to the thinness of the substrate on which the patch is fabricated. To broaden bandwidth, electromagnetically coupled patches (EMCP) can be employed which include, for example, a coupling patch on a first substrate and an antenna patch on a second substrate, the coupled patches being substantially parallel and separated by a particular distance. The greater the separation distance, the greater the increase in bandwidth. Bandwidth is further increased by selecting a material to fill the separation distance which has a low dielectric constant (i.e., ideally 1=the dielectric constant of air). Such material should preferably provide structural rigidity to insure uniform EMCP spacing, and should be lightweight.
One method to enhance the purity of circular polarization of patch antennas (i.e., to reduce the axial ratio) is to connect a plurality of complimentary patches to a feeding network in sequential rotation whereby there is a uniform angular spacing of the feeding points between the patches. In this fashion, the orientation of the radiation from each patch is rotated relative to the orientation of the radiation from complementary patches. Furthermore, the feeding network should preferably provide a uniform phase difference between the signals sent to or received from the patches. For example, in a four patch arrangement, the signal fed to the first patch has a particular phase relationship with respect to the feedline; the signal fed to the second patch lags by 90.degree. the signal fed to the first patch; the signal fed to the third patch lags by 180.degree. the signal fed to the first patch and lags by 90.degree. the signal fed to the second patch; and the signal to the fourth patch lags by 270.degree. the signal fed to the first patch, lags by 180.degree. the signal fed to the second patch, and lags by 90.degree. the signal fed to the third patch. In addition, the location of the feeding point on each patch is correspondingly rotated 90.degree. so that the feed point of the second patch is rotated 90.degree. with respect to the feed point of the first patch; the feed point of the third patch is rotated 90.degree. with respect to the feed point of the second patch and 180.degree. from the feed point of the first patch; and, the feed point of the fourth patch is rotated 90.degree. with respect to the feed point of the third patch, 180.degree. from the feed point of the second patch and 270.degree. from the feed point of the first patch.
A larger number of feed patches can be used as long as the signal phases and feed locations are uniformly distributed around 360.degree.. Ideally, the combined radiation from all of the patches would have perfectly circular polarization (i.e., OdB axial ratio). In actual practice, of course, such perfect circular polarization has not been achieved.
Heretofore, hybrids have often been employed to phase shift the signal fed to (or from) the patches in a sequential rotation network. The use of such hybrids in a feeding network may consume so much space, however, that in many applications with space constraints the feeding network may have to be situated on a separate substrate and coupled directly or electromagnetically to the microstrip patch (which can be an antenna patch or, in the case of EMCP, a coupling patch). As can be appreciated, this increases the complexity and cost of the antenna and tends to reduce its efficiency. If fewer patches are used, or if the same number of patches are used but they are spread out over a larger area, space may be available for the hybrids but the radiation pattern may have excessive grating lobes resulting in reduced efficiency and degraded coverage characteristics. If more patches are used, or if the same number of patches are used but are placed closer together, coupling between patches may seriously degrade antenna performance.
It is desirable, therefore, to provide an antenna having high purity circular polarization (i.e., a low axial ratio), substantially uniform coverage, broad bandwidth and high efficiency, and which is easy and inexpensive to fabricate. It is further desirable for such an antenna to be small, lightweight and to be fabricated from space qualified materials so as to be well-suited for use in a satellite. It is also desirable that the material used between substrates in an EMCP pair have a low dielectric constant, be lightweight and rigid, and to provide for substantially uniform spacing between the substrates.